strange MTA Relay Problem

I will send my outgoing mails over the provider. The providers server needs no authification and can send mail up to 10 MB. So i add the url in the setting.

Now the problem: Little E-Mails < 10KB are no Problem to send. But by Larger i receive the fallowing message:

(conversation with smtp.isp.ch[??.?.??.??] timed out while sending message body)

If i sent greate files from a other host direkt to the smtp.isp.ch server, ther is no problem with the same mails.
So I testet the mailing on the zimbraserver with telnet. And now i have see that by some mails its not possible to end the data with [Enter].[Enter]. And then i must wait until the server send a timed out. But only by greate Mails or perhaps if they have special characters.

Has someone a solution for this problem?

Can it be that it is a problem with UTF-8 and the other charsets? Where can i change the charset?